As a gas sensor for detecting the concentration of oxygen in an exhaust gas, there is conventionally known a sensor including an oxygen ion-conducting gas sensing element and mounted for use on an exhaust pipe of an internal combustion engine of a vehicle (see, for example, Patent Documents 1 to 3).
Herein, a general explanation will be given to a gas sensor having the same configuration as those of Patent Documents 1 to 3 (see FIG. 6). In the gas sensor P5, a gas sensing element P1 is fixed in a metal shell P2 with a rear end part of the gas sensing element P1 being covered by a metallic outer tube P3 and by a metallic protection tube P4.
The gas sensing element P1 has an element body (sensor element) made of an oxygen ion-conducting solid electrolyte and at least an inner electrode P7 located inside the element body.
In this type of gas sensor P5, a separator P6 is located rear of the gas sensing element P1 and fixed in position by a rear end portion of the outer tube P3; a metal terminal P8 is placed in contact with the inner electrode P7; and a heater P9 is located inside the gas sensing element P1 and pressed at a front end portion thereof against an inner circumferential surface of the gas sensing element P1 so as to heat the gas sensing element P1.
As mentioned in Patent Documents 1 to 3, the solid electrolyte, which can be formed from a ceramic material such as zirconium oxide, is used as the material of the element body of the gas sensing element P1 (sensor element). In order for the gas sensing element P1 to perform its oxygen sensor function, the gas sensing element P1 needs to be heated to an activation temperature. It is thus common practice to utilize the heater P9 such that the element body of the gas sensing element P1 (sensor element) can be heated to the activation temperature by the heater P9.
There is also a need to raise the temperature of the element body of the gas sensing element P1 (sensor element) to the activation temperature in a short time, that is, a need to allow early activation of the element body of the gas sensing element P1 (sensor element) when the oxygen concentration needs to be detected accurately in a short time e.g. at the time of starting of an internal combustion engine. If the heater P9 is spaced apart from the element body of the gas sensing element P1 (sensor element), it takes time until the heat generated by the heater P9 is conducted to the element body of the gas sensing element P1 (sensor element). This results in a longer time required for activation of the element body of the gas sensing element P1 (sensor element). It is conceivable to bring the heater P9 (more specifically, a heating portion of the heater P9) into direct contact with the element body of the gas sensing element P1 (sensor element) so as to reduce the time required for activation of the element body of the gas sensing element P1 (sensor element).